Isotopic analysis of reservoir fluids in coal seams is a method to evaluate the type of chemical process producing coal bed methane gas retrieved from those seams (Dudley D. Rice, George E. Claypool, AAPG Bulletin, Volume 65, Issue 1. (January), Pages 5-25 (1981)). This method is possible because methanogenic consortia that digest the coal to produce methane preferentially digest moieties containing 12C atoms. Thus, gases produced by methanogens are enriched in 12C.
The preferential metabolysis of 12C from coals or shales during biogenesis results in enrichment of 13C in the carbon containing salts and other materials residual to the process. This enrichment has been used to typecast fluids from particular coal seams (Enhancing Microbial Gas From Unconventional Reservoirs: Geochemical And Microbiological Characterization Of Methane-Rich Fractured Black Shales, FINAL REPORT (March 2004-September 2004) Prepared by: Anna M. Martini, Klaus Nüsslein, Steven T. Petsch, Prepared for: RESEARCH PARTNERSHIP TO SECURE ENERGY FOR AMERICA, Subcontract No. R-520) and even to track the eventual impact of the fluids on the environment (http://faculty.gg.uwyo.edu/cfrost/pdfs/2010%20Frost%20CBNG%20book.pdf). But the enrichment of 13C in the carbon containing salts and other materials residual to the preferential metabolysis of 12C from coals or shales during biogenesis has not been used in the manner of the present disclosure, that is, to measure hydrologic constraint. These measurements are made while operations may be undertaken that unintentionally or intentionally change the constraint in a reservoir. More specifically, “hydrologic constraint” refers to the extent to which the fluids resident in a coal seam or shale formation are residual from the methanogenic process. Thus, highly constrained reservoirs have little or no influx of foreign fluids that are not residual from the methanogenic process. Highly unconstrained reservoirs have appreciable, and sometimes significant, influx of foreign fluids that are not residual from the methanogenic process. Such foreign fluids can include, but are not limited to, fluids from other geological formations, fluids from surface waters, fluids from recharge at formation outcrop, and others. Influx of such fluids can occur by a variety of mechanisms including, but not limited to, via a wellbore connecting multiple formations, via a multi-formation geological fault, and via fracture networks connected to similar cross formation flow pathways.
Coal is the result of coalification, which pertains to the degree of biogenic and then thermogenic transformation of organic sediments. Coalification occurs initially via methanogens. Methanogens perform methanogenesis, a type of biogenic process, which results in enrichment of 13C isotope because 12C is consumed as it is easier to metabolize. Thermogenesis however, does not affect 13C levels to a significant extent. Moreover, surface water generally has non-enriched 13C levels. Gases produced by thermogenic processes, on the other hand, undergo no such kinetic preferential metabolysis and therefore are not significantly enriched in 12C. As a result, measurement of 12C levels in methane gas produced from coals can indicate whether the coal is undergoing biogenic or thermogenic coalification.
The natural abundance of 12C isotopes is about 98.9% of carbon atoms. The natural abundance of 13C isotope is about 1.1%. In water or an aqueous media, the dissolved inorganic carbon (DIC) consists of carbon dioxide/carbonic acids [CO2+H2CO3], bicarbonate [HCO3−] and carbonate [CO32−]. Therefore, the tracer of interest is δ13CDIC, which is defined as the 13C/12C ratio in the sample divided by the standard 13C/12C ratio, minus one, and typically expressed in parts per thousand by then multiplying the result by one thousand. In coal beds, bacteria ferment acetate (CH3COOH→CH4+CO2) and reduce carbon dioxide (CO2+4H2→CH4+2H2O). As a result, bacterial methanogenesis preferentially removes 12C and the remaining DIC is enriched in 13C.
A positive δ13CDIC identifies groundwater in which biogenic production of methane has occurred. The δ13CDIC of groundwater that is not associated with methanogenesis have ratios of negative 5% to negative 20%, whereas water coproduced with coal bed methane has ratios as high as +35%.
Therefore, there is a need in the art for methods using δ13C enrichment measurements in coal and shale fluids to indicate practical reservoir characteristics of commercial importance, such as hydrologic constraint in aquifers. There is also a need for sensor devices that can perform such measurements rapidly and effectively in coal bed and shale fluid reservoirs. The present disclosure was made to address this need.